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Hindawi Publishing CorporationApplied and Environmental Soil ScienceVolume 2013, Article ID 601058, 7 pageshttp://dx.doi.org/10.1155/2013/601058

Research ArticleA Case of Cyperus spp. and Imperata cylindrica Occurrences onAcrisol of the Dahomey Gap in South Benin as Affected bySoil Characteristics: A Strategy for Soil and Weed Management

Brahima Kone,1 Guillaume Lucien Amadji,2 Amadou Toure,3

Abou Togola,3 Mariame Mariko,3 and Joël Huat3,4

1 Felix Houphouet Boigny University, Soil Science Department, 22 BP 582, Cocody, Abidjan 22, Cote d’Ivoire2 University of Abomey-Calavi, Departement of Agronomy Sciences, BP 499, Calavi, Benin3 Africa Rice Center, BP 2031, Cotonou, Benin4CIRAD, UPR Hortsys, 34398 Montpellier Cedex 05, France

Correspondence should be addressed to Brahima Kone; [email protected]

Received 7 October 2012; Revised 15 April 2013; Accepted 28 April 2013

Academic Editor: Philip J. White

Copyright © 2013 Brahima Kone et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Because of the limiting efficacy of common weed control methods on Cyperus spp. and Imperata cylindrica their occurrences intropical agroecologies and the effect of soil properties in suppressing these species were investigated in south Benin (Cotonou),a typical ecology of the Dahomey gap. Weeds and soil samples were collected twice early and later in the rainy season in 2009 atfour topographic positions (summit, upper slope, middle slope, and foot slope). Sampling was done according to Braun-Blanquetabundance indices (3 and 5) and the absence (0) of Cyperus and Imperata in a quadrat, respectively. The relationship between theirrespective abundances and soil parameters (texture, C, N, P, K, Na, Ca, Mg, and Fe) was explored.Weed occurrence was less relatedto soil texture, and Imperata growth was more influenced by soil nutrients (K, Ca, and Fe) than Cyperus spp. Soil cation ratiosof K :Mg and Ca :Mg were the main factors that could be changed by applying K and/or Mg fertilizers to reduce Cyperus and/orImperata occurrence. Maintaining high Fe concentration in soil at hillside positions can also reduce Imperata abundance, especiallyin the Dahomey gap.

1. Introduction

Weeds are notorious yield reducers that are, in many sit-uations, economically more important than insects, fungi,or other pest organisms [1, 2]. The yield loss due to weedsis almost always caused by an assemblage of different weedspecies, and these can differ substantially in competitiveability [3]. In rice growing agro-ecologies of West Africa,weed species assemblages include nutsedges and speargrassthat are perennial and serious threats [4].

Imperata cylindrica (L.) Raeuschel (speargrass) is a com-mon and persistent weed in upland ecology. It reproducesthrough seeds and rhizomes. This weed is particularly diffi-cult to control, as it is tolerant to fires and shallow cultivationdue to its extensive underground network of rhizomes.The weed tends to be abundant where fields are regularly

cultivated and burnt, as it recovers rapidly from disturbance,and burning induces flowering. It exerts great competition oncrops [5, 6].

Inmoist to hydromorphic upland areas, some of themostintractable weed problems in rice are due to the perennialsedges Cyperus rotundus L. (purple nutsedge) and Cyperusesculentus L. (yellow nutsedge). Their tubers and seeds canremain dormant to survive periodic flooding or dry seasons.These species are able to multiply rapidly through tuberswhich can be greatly accelerated by soil tillage [7]. Becauseof these characters, I. cylindrica and Cyperus spp. are typicalweeds of intensively cultivated lands and very difficult tocontrol [8].

Some relative successes were observed using chemicalmethods for the control of Cyperus spp. and I. cylindrica[9]. But, herbicide is expensive and not always available to

2 Applied and Environmental Soil Science

West African smallholder farmers. Moreover, there is a riskof environmental pollution.Therefore, additional knowledgeand technology are needed for improving the control ofCyperus spp. and Imperata cylindrica in Africa, especially inthe Dahomey gap (south Benin) where they are seriouslythreatening livelihoods [10].

Considering that reduced soil fertility often increasesweed infestation [11] and the effects of soil parameters inweedoccurrence in Nigeria [12], the concept of chemotropism [13]is hypothesized in order to identify soil nutrients (N, P, K,Ca, Mg, and Fe) that influence the occurrence of Cyperusspp. and /or Imperata cylindrical attention should be paid totopography and soil texture influencing soil organic matterthat can change vegetation structure [14].

A nonsite replicated ecological study [15] was initiatedduring the rainy season of 2009 along a catena of Acrisol insouth Benin (West Africa), which is a typical ecology of theDahomey gap. The implication of soil texture and nutrients(C, N, P, K, Ca,Mg, and Fe) on the occurrence ofCyperus spp.and I. cylindrica in this upland agro-ecology was explored.The aim was to identify soil parameters that can be usedto develop a management strategy for the control of theseperennial weeds in continued rice growing agro-ecologies ofthe Dahomey gap.

2. Materials and Methods

2.1. Site Description. The study was conducted during the2009 cropping period (June–September) at the Africa RiceCenter (ex-WARDA) experimental station in Cotonou (6∘ 28N; 2∘ 21 E, 15m asl), Benin.The site is a derived savanna zonein the Dahomey gap of West Africa. The rainfall pattern wasbimodal with 807mm during the cropping season of 2009.The soil is locally named “terre de barre.” It is a very deepAcrisol (>10m), with sandy top soil (0–40 cm) and free ofconstraints (gravels, stones, or hardpan) to plant rooting inthe profile. Soil pHwater was 5 and C :N, Ca :Mg and K :Mgratios were about 12, 1.7 : 1 and 1 : 1, respectively. The studiedarea is continuously cultivated for upland rice production.

2.2. Weed Sampling. Two dominance-abundance indiceswere considered for Cyperus spp. and Imperata cylindrica asa proportion of weeds in 1m2 quadrats: 3 = covering between1/4-1/2 of the soil surface and 5 = covering more than 3/4 ofthe soil surface [16]. Weeds were also sampled where Cyperusand Imperatawere absent, and zero (0)was the correspondingindex. Indices 0, 3, and 5 were considered as absent (A),medium density (M), and high density (H), respectively.Three sampling places (A, M, and H) were identified for eachof Cyperus spp. and I. cylindrica in the summit, the upperslope, the middle slope, and the foot slope as described byRuhe and Walker [17]. Sampling was replicated twice in thestudied site: in July (1) and in September (2) at the beginningand the end of the rainy season, respectively. In a quadrat, allthe weed species were sampled separately with bellow grounddrymatter. Individual identificationwas done as described byAkobundu and Agyakwa [18] and they were coded accordingto Braun-Blanquet [16]. The roots were cut off, and weedswere oven dried at 70∘C for 24 hours before weighting.

2.3. Soil Sampling and Analysis. Topsoil (0–20 cm depth) wassampled using augur (20 cm × 7 cm) in each quadrat at everytime of weed sampling. After sampling weeds, a soil samplewas taken from the centre of the quadrat. Twenty-four (24)soil samples (12 at each sampling time) were taken, andsun dried before laboratory analysis. Soil particle sizes weredetermined by the Robinson pipette method [19] as wellas organic-C, total-N, P-available (BrayI), and total-P (Pt).Soil exchangeable K, Ca, Mg, and extractable Fe were alsodetermined. Chemical analyses were performed as describedby Page et al. [20].

2.4. Statistical Analysis. Descriptive statistics were used tocalculate the frequency of weed species in a specific quadrat.By analysis of variance (ANOVA),mean values of soil particlesize and nutrients were determined for each topographic sec-tion. Mean values of total weed biomass dry matter were alsodetermined for each placement of quadrat. The mean valueswere separated by Student-Newman and Keul methods. Therelationships between biomass dry matter and soil physicaland chemical characteristics were evaluated by Pearson cor-relation. SAS 10 package was used for these analyses. Soil clay,sand, C, N, Pa, K, Ca, Mg, and Fe concentrations were usedto discriminate Imperata abundance indices (3 and 5) andabsence (0) by canonical function analysis using SPSS 16.

3. Results

3.1. Soil Characteristics. Thetopsoil (0–20 cm)was sandy, andthe middle slope (MS) had a significantly greater proportionof sand than other positions (Table 1). This topographicposition also had significant lower contents of clay (11%) andsilts (4%). The soil had moderate carbon content throughoutthe toposequence except in the soil at foot slope (FS)position; where it was significantly higher, similar gradientwas observed for total nitrogen-Nt content along the topose-quence. Phosphorus (P-available and P-total) contents weremoderate in the soils of upper slope (US) and MS while theywere higher in the soils at summit (SUM) and FS positions.Soil concentrations of divalent cations (Ca and Mg) weresignificantly higher at the SUM position with a decreasingtrend along the toposequence. Similar result was observedfor soil Fe concentration. Meanwhile, monovalent cations (KandNa) concentrations contrasting with these results with anincreasing trend from the SUM to the FS.

3.2. Weed Assemblages. Seventeen weed species were mostfrequently encountered in the quadrats. Other species werealso observed in at very low frequencies, and their cumulativefrequencies were depressed in themedium and high densitiesof Cyperus spp. and I. cylindrical, respectively (Figure 1).However, more diverse communities were observed whereCyperus spp. and I. cylindrica were recorded in the quadratscompared to places where they were absent. Nevertheless,Richardia brasiliensis (Ricbr) was encountered in all quadratswithmoderate (9.09%) to high (21.74%) frequency andDacty-loctenium aegyptium (Dacae) at a low frequency (2.56%–8.82%), about 14% in 1m2 characterized the medium densityof Cyperus spp. and I. cylindrica occurrence, respectively.

Applied and Environmental Soil Science 3

Table 1: Mean values of soil particle sizes and nutrient contents (C, Nt, Pa, Pt, Mg, K, Ca, Na, and Fe) along the toposequence in topsoil(0–20 cm).

Sand(%)

Clay(%) Silt (%) C-org

(%) Nt (%) Pa(ppm)

Pt(ppm)

Mg(cmol kg−1)

Ca(cmol kg−1)

K(cmol kg−1)

Na(cmol kg−1)

Fe(ppm)

SUM 82.0b 12.5ab 5.5b 1.19b 0.07b 26.4a 170.8b 2.8a 2.9a 0.2c 0.05c 2800a

US 81.0b 14.0a 5.0c 1.19b 0.06c 12.8b 126.0c 2.5b 2.6b 0.1d 0.10c 1960b

MS 85.0a 11.0b 4.0c 1.09b 0.05d 8.0b 117.0c 2.0c 2.1c 0.3b 0.21b 1924b

FS 74.0c 14.0a 11.5a 1.71a 0.08a 25.2a 241.0a 2.0c 1.9d 0.4a 0.55a 1547b

Pr > 𝐹 <0.0001 0.001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001

0

10

20

30

40

50

60

Freq

uenc

y (%

)

Weed species in quadrate placement

Amahy

Casro

Cleci

Comer

Cypes

Cypro

Cypsp

Dacae

ImpcyMalop

Maral

Oldco

Oldhe

Ricbr

Taltr

Tripr

Versi

Other

CyperusCyperus Imperata Cyperus ImperataImperataAbsent Medium density High density

Figure 1: Weed specie average frequency in different placements of the quadrate.

Three species of Cyperus including Cyperus esculentus, Cype-rus sphacelatus, and Cyperus rotundus were identified. Butonly Cyperus rotundus (Cypro) was encountered for themedium density of Cyperus spp. Whenever high densitiesof Cyperus and Imperata were observed, they accounted for25.92% and 20.51%, respectively. Commelina erecta (10.26%)and Richardia brasiliensis (12.82%) were the most frequentweed species associated with the high density of Imperata.

Figure 2 shows themean values of total weed biomass drymatter for each quadrat according to Imperata and Cyperusdensities. Weed biomass was significantly higher in quadratswith high densities for Imperata and Cyperus, respectively.Wherever Cyperus and Imperata were absent, total weedbiomass did not differ significantly with that observed fortheir medium densities.

3.3. Weed and Soil Relations. Table 2 shows the Pearsoncorrelation values (R) between soil characteristics and thebiomass of Cyperus and Imperata, respectively, when high

a

a

b

b

abb

0

5

10

15

20

25

Quadrate placements for weed species

Wee

d dr

y m

atte

r (g m

−2)

Imperata Cyperus

AbsentModerate densityHigh density

Figure 2: Weed biomass dry matter in quadrate placements fordifferent densities of Imperata and Cyperus (a and b are indicatingmean values that are different significantly).


Table 2: Pearson correlation coefficient values between soil charac-teristics and the dry matter of Cyperus and Imperata in their respec-tive high density quadrate placement.

𝑅

Cyperus biomass Imperata biomassClay −0.31 −0.66

∗

Silt −0.19 0.62∗

Sand 0.22 −0.02

C −0.34 0.39N −0.39 −0.08

Pa 0.55 0.48Pt −0.13 −0.13

Mg −0.11 −0.60

K 0.06 0.73∗∗

Ca −0.36 −0.84∗∗

Na −0.29 0.54Fe 0.03 −0.88∗∗∗significant at 10%; ∗∗significant at 1%.

Func

tion

2

53

0

1050−5−10−15

Function 1Fe, Pa

Sand, N, K

Clay, sand,Pa, Mg

10

5

0

−5

−10

C,Ca

Group centroid5

30

Indice

Figure 3: Canonical function discrimination of Imperata abun-dance according to soil parameters.

densities were observed. Carbon-C (−0.34), N (−0.39), Pa(0.55), and Ca (−0.36) had highest correlation values withCyperus biomass, but these results were not significant.Meanwhile, highly significant (𝑃 < 0.01) correlations wereobserved between the biomass of Imperata and K (0.73), Ca(−0.84), and Fe (−0.88) concentrations in the soil. Moreover,clay (−0.66) and silt (0.62) had significant correlation withImperata biomass at certain level of probability (𝑃 = 0.1).

Figure 3 shows that group centroids (mean of the dis-criminant score for a given category of dependant variable)

Table 3: Mean values of clay, silt, Fe, K, and Ca contents in soil atdifferent topographic positions for different abundance of Imperata.

Clay Silt Fe K Ca% (ppm) cmol kg−1

SUMA 9c 5b 3277.87b 0.02b 3.32a

M 10b 6a 1506.75c 0.01c 2.84b

H 14a 6a 3509.50a 0.04a 3.28a

𝑃 > 𝐹 <0.0001 0.006 <0.0001 <0.0001 <0.0001

USA 14b 5b 2584.94a 0.05b 2.87a

M 12a 4c 654.08c 0.03c 2.90a

H 13b 8a 2330.99b 0.08a 2.88a

𝑃 > 𝐹 <0.0001 <0.0001 <0.0001 <0.0001 0.272

MSA 10a 4b 2366.19a 0.18c 2.00c

M 7c 5a 1018.08c 0.30a 2.19b

H 9b 3c 2046.45b 0.23b 2.44a

𝑃 > 𝐹 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001

FSA 10a 9c 591.65c 0.36b 2.87a

M 10a 11a 4498.86a 0.41a 2.02c

H 9b 10b 1269.39b 0.21c 2.25b

𝑃 > 𝐹 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001SUM: Summit; US: upper slope; MS: middle slope; FS: foot slope; A: absent;M:moderate density; H: high density; a,b,care indicatingmean values that aresignificantly different.

of Imperata abundance indices are well separated, attestingthe ability of soil parameters (clay, sand, Pa, C, N, K, Ca, Mg,and Fe) to discriminate Imperata density: absence wasmainlycharacterized by lower Fe and Pa concentrations in soil, whileincreasing of soil sand, clay, Pa, and Mg was characterizedby abundance (5). The medium (3) density of Imperata wasobserved for low soil C and Ca contents.

Table 3 shows variance of these relationships accordingto topographic positions. In fact, the highest soil clay (14%),Fe (3509.5 ppm), and K (0.04 cmol kg−1) concentrations wereassociated with the abundance of Imperata at the summit.However, the absence of Imperata at the US andMS positionswas more related to high soil Fe concentration. Highest (US)and lowest (MS) K concentration in soil characterized thehighest density and the absence of Imperata, respectively,according to topographic positions. Soil calcium concentra-tions were not significantly different for the absence and thehigh density of Imperata at the SUM and the US. In contrast,the highest (2.44 cmol kg−1) and lowest (2.00 cmol kg−1) soilCa concentrations were related to high density of Imperata atthe FS and MS positions, respectively.

4. Discussion

4.1. Extension and Limit of Our Finding. Cyperus esculentus,Cyperus sphacelatus, and Cyperus rotunduswere encounteredin the studied area. The last species was the most frequentalong the toposequence compared to the others. However,the occurrence of Cyperus sphacelatus contrasts with obser-vations by Johnson [5] who mentioned the occurrenceof Cyperus difformis in rice agro-ecosystems rather than


C. sphacelatus. Dactyloctenium aegyptium (Dacae), Richardiabrasiliensis (Ricbr), and Commelina erecta (Comer) were themost frequent weed species associated with Cyperus andImperata according to our study which differs from previousknowledge of weed community in upland ecology of WestAfrica [8]. Furthermore, Andropogon, Cymbopogon, Hypar-rhenia, Pennisetum, and Setaria were frequently encounteredin the southern guinea savanna including Brachiaria spp.instead of Setaria identified in the northern guinea savannaof West Africa [21]. Therefore, the studied location in theDahomey gap differs from the ecologies of north and southguinea savanna of West Africa. Indeed, the studied zone wasdescribed as a costal savanna [22] representing a marginalzone ofWestAfrica, whichwas named theDahomey gap [23].Therefore, this particularity could have induced differences inweed assemblages compared to the others savanna ecologies.Hence, our analyses provide additional knowledge of weedoccurrence in rice growing agro-ecologies of West Africa.

The studied ecologywas also characterized by a landscapeof a wide plateau with depressions instead of a hydrographicnetwork as described by Raunet [24] on the crystallinebasement of West Africa. However, the trends of variation insoil nutrients were the same along the toposequence of terrede barre as described elsewhere [25]: divalent cations con-centrations decreased from the summit to the foot slope,while soil concentrations of monovalent cations increased.Therefore, the influence of soil parameters on weed speciesoccurrence as revealed in our actual study can be consideredfor the entire Dahomey gap and the other ecologies of WestAfrica according to the rule of nonreplicated ecological study[15].

4.2. Weeds Control by Soil Fertility Management. The lowesttotal weed biomass was observed in the quadrats whenCyperus and Imperata were absent. This result attested thatfactors affecting the occurrence of these species can also beconsidered for the others weeds species. Therefore, our resultprovides an insight to deeper knowledge of the interactionsof divers weed communities with soil. However, exceptionsshould be considered for Richardia brasiliensis (Ricbr) andDactyloctenium aegyptium (Dacae) which were encounteredin all the quadrats (Figure 1) indifferently to the density ofCyperus and Imperata.

No significant relationship was observed between thesoil parameters studied and Cyperus spp. biomass (Table 2).However, Kone et al. [26] showed a decreased of Cype-rus abundance with the increasing soil Mg concentration.Indeed, our study revealed some implications of this nutrient(Mg) in Cyperus occurrence through the soil balance of Kand Mg, excepted in the FS position (Table 4). Applicationof K fertilizer might be able to increase soil K :Mg ratio anddecreaseCyperus abundance at the SUM, while supplyingMgmight induce the same effect on Cyperus occurrence at theUS and MS positions by altering the Ca :Mg ratio. Therefore,K or Mg amendments are required for depressing Cyperusoccurrence according to topographic positions. Indeed, soilbalance in K :Mg and Ca :Mg can affect not only K and Caavailability to plant but also P as observed by Yates [27].Thus, it appears that Cyperus occurrence is more related to

Table 4: Pearson correlation values (𝑅) and its probabilities (𝑃)between Cyperus spp. and Imperata Cylindrica and soil differentratios in K :Mg and Ca :Mg at each topographic position.

Correlation (𝑅) and probability value (𝑃)Cyperus spp. Imperata Cylindrica𝑅 𝑃 𝑅 𝑃

K :Mg

SUM −0.703 0.0002 0.397 0.021US 0.853 <0.0001 0.482 0.007MS 0.956 <0.0001 −0.338 0.032FS 0.273 0.290 0.378 0.034

Ca :Mg

SUM 0.843 <0.0001 −0.231 0.195US 0.839 <0.0001 0.066 0.729MS 0.822 <0.0001 −0.839 <0.0001FS −0.094 0.719 −0.743 <0.0001

SUM: Summit; US: upper slope; MS: middle slope; FS: foot slope.

imbalanced ratio of nutrients in soil rather than the depletioneffect of sole nutrient content.

I. cylindrica occurrence was significantly influenced bysoil Fe, Ca and K concentrations with less importance tosoil particle sizes (Table 2). This result restricts the assertionmade by Andreasen and Streibig [28] concerning the influ-ence of soil texture on weed occurrence. Furthermore, theinfluence of soil nutrients was confirmed partially accordingto topographic positions. Highest soil K concentration wasassociated with high density of Imperata at the uphill position(SUM and US). Instead of reducing soil K concentrationfor depressing the density of Imperata, management strategymust focus on reduction of K :Mg by supplying Mg com-pound as consequence of Pearson correlation value observedin Table 4. Imperata density can also decrease at the downhillposition with the decrease or increase of soil Ca concen-tration at the MS and the foot slope positions, respectively.However, no consistent management of soil Ca can be drawnfrom our study regarding the contrasts observed in Tables 3and 4. Further investigations should also focus on K : Ca:Mgor [(Ca + Mg) : K] ratio for improving knowledge of theinteraction between Ca and Imperata. Up to now, our studyhas improved knowledge of the effect of soil nutrients balanceon weed occurrence as mentioned by the Midwest Organicand stainable Education Service (MOSES) [29].

4.3. Indicators of Soil Degradation in the Environment. Thestudy revealed that highest soil Fe concentrations at theHillside (US and MS) were also associated with a lowdensity of Imperata. Otherwise, Fe leaching as observed indegraded soil [30] may be favorable to Imperata occurrencedepending on topographic positions. Low soil K (0.03–0.08 cmol kg−1) concentrations observed at the US positionreinforced (Table 3) this assertion. Indeed, low K concen-trations in soils of Africa occur generally in degraded soilsaccording to Juo and Grimme [31]. Therefore, our findingconfirms the fact that Imperata is a bioindicator of degradedsoil as propounded by Scherr and Yadav [32].

The leaching of soil K and Mg leads to impoverishmentof soil in these nutrients, justifying their requirement for


the control ofCyperus spp. occurrence, especially by changingsoil nutrient balance.Thus, soil chemical degradation can alsoinduce Cyperus spp. occurrence and likewise for Imperata. Inthe Philippines, Imperata cylindrica and Cyperus compressuswere observed in strongly weathered soil [33] corroboratingour analysis and suggesting that soil organic amendment canreduce the invasion of these species, especially for Cyperus[26]. In the context of land management, we can recommendfurther study of fallowing or the cultivation of cover cropson Cyperus spp. and Imperata occurrences on Acrisols.These practices might be able to avoid soil degradation [34],restricting Cyperus and Imperata invasions.

Most of the knowledge of upland soil chemical degra-dation processes is related to nutrient depletion [35, 36].Nutrient ratios have been investigated less frequently. Exceptfor soil C (1.09–1.71%) and N (0.05–0.08%) concentrationsand K (0.1 cmol kg−1) concentrations at certain levels of sig-nificance, the studied soil had high nutrient concentrations,especially for P (8–26.4 ppm) and Na that reached 0.55 cmolkg−1 (Table 1). But rice yield can drop to 0.26 tha−1 evenfor improved varieties with a potential yield of 4-5 tha−1,as a consequence of mineral imbalances, particularly C :N,Ca :Mg and K :Mg [23]. Regarding yield reduction as anindicator of agricultural soil degradation [32], we deduce thatunbalanced soil nutrients in the studied area are likely tocontribute.These characteristics (nutrient ratios) of soil werealso involved in rice P-nutrition in an acid Ferralsol ofNigeria[37]. More investigations for understanding the effects ofnutrient ratios in soils are required in tropical ecologies forimprovement of agricultural land use, weed management,and restoration of degraded soils.

Our results showed association of high density of Imper-ata to Commelina erecta and Richardia brasiliensis. Finally,Imperata, Cyperus, Commelina, and Richardia can be con-sidered as indicators of degraded Acrisols in the studiedenvironment. The importance of soil exchangeable cationconcentrations (Ca, Mg, Fe, and K) and the ratios of Ca :Mgand K :Mg for weeds occurrence in the Dahomey gap, andWest African ecosystems by extension, confirms the workdone by Udoh et al. [12] citing the importance of soil C, N,Zn, and Mn contents.

5. Conclusion

Soil parameters have influence the occurrence of weedsaccording to topographic positions, especially for Cyperusspp. and Imperata Cylindrica. However, the relationship withCyperus spp. was less pronounced than with Imperata Cylin-drica.

Soil balance, and in particular K :Mg and Ca :Mg ratios,were the main factors affecting the occurrence of these weedsin the studied ecosystem. Applying Mg and/or K fertilizersmight be employed to change these soil characteristics toreduce Imperata and Cyperus invasions. Soil Fe concentra-tion also influenced Imperata occurrence. It is suggested thatCyperus and Imperata occurrences prevailed in degradedsoil for which they can be used as indicators, along withCommelina and Richardia. To some extent, our finding canbe extended to other West African ecosystems.

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